The combustion of fuel in a diesel or Otto engine results in production of various pollutants such as heavy hydrocarbon particles, carbon monoxide, nitrogen oxides (NOx) and carbonaceous particulate matter. Emissions control requirements mandate low levels of emissions.
Among exhaust gas cleaning systems, NOx storage catalytic converters are well known. This type of catalytic converter is comprised of two material components. The first serves for adsorbing and storing NOx during lean operation of the engine. Lean operation is when the combustion air ratio lambda (λ) is greater than 1, that is, if there is hyperstoichiometric combustion, in which there is residual oxygen in the exhaust gas.
The second material component is a catalytic material with three-way characteristics. In order to regenerate the storage catalytic converter, a reducing exhaust gas with high reducing agent content is needed to cause release of Nx stored in the first component and reduction of the NOx to molecular nitrogen (N2) and oxygen (O2) in the presence of the catalyst. An internal combustion engine produces reducing exhaust gas when there is “rich” combustion, that is to say, hypostoichiometric combustion with λ<1, in which there is no or only a little residual oxygen in the exhaust gas.
Diesel engines preferably operate with excess oxygen (λ>1) over the entire characteristic map area. However, for the above discussed reasons, in order to be able to use an NOx adsorber system the diesel engine must be changed over from time to time from lean to rich operation and back again. Such systems are described for example in DE 195 43 219 C1. However, programming engine operation for the required relationship between the lean operating phase and the rich operating phase in the combustion control of the vehicle engine is complex. Further, software must be tailored to each version and modification of the engine and associated equipment. There is a need for a simple system for catalyst regeneration, capable of being operated without interfering with the operation of the engine and capable of operating indepenent of the condition of the engine.
Another problem with storage catalytic converters is that in some converters a lean-rich cycle with respectively equal durations of a lean and rich motor operating phases (for example 60 seconds each) is required. This results in inefficient engine operation.
Yet another problem with catalytic converters is that catalysts are most effective at operating temperatures of 300° to 400°. During cold start-up of the engine the catalyst is inefficient, NOx emissions are high, and emissions standards may not be met. At low load conditions or at idle, exhaust temperatures may only reach 200° C. At this low exhaust gas temperature the catalytic converter is cool and ineffective. It is known to increase exhaust gas temperature by retarding ignition timing in gasoline engines or retarding fuel injection timing in diesel engines (since heat of combustion not converted into mechanical work is vented to the exhaust), but the resulting increase in temperature is limited. Further, retarding timing reduces engine power, hampering engine operability.
It is also well known to reduce emissions of particulates—flammable fine particles which in diesel engines are the main cause of black smoke in exhaust gas. For this, particulate traps are provided upstream of catalytic converter. Diesel particulates collected in the particulate trap undergo self burning at 500° C., or 350° C. in the presence of an oxidation catalyst. Exhaust temperature can reach 500° C. only at high speeds or under high loads. Under typical operating conditions such high exhaust temperatures are not experienced for sufficient time periods to clear the trap. Thus, particulates accumulate and and eventually the trap becomes plugged, increasing backpressure. For this reason, systems for artificially increasing temperature upstream of the particle trap are known and are referred to as auxiliary or “regenerative” burners. Burners in the exhaust pipe just upstream of the particulate trap are disclosed for example in U.S. Pat. Nos. 4,567,725, 4,677,823 and 5,826,428. However, it is difficult to maintain stable combustion at low pressures. Further, these systems suffer from disadvantages associated with cost, safety and durability. For cost and weight reasons burners tend to be made of relatively thin sheet metal, and for engineering reasons they tend to be located near to the vehicle fuel tank. With a limited explosive containment rating, the danger of locating such a thin walled burner near a vehicle fuel tank is readily apparent. Further, parking a vehicle with a hot burner in the exhaust pipe on a flammable surface such as dry grass is dangerous.
It is thus an object of the invention to provide a system and device for regeneration of catalyst and also for regeneration of the particulate trap which does not interfere with the lean operation (λ>1) of a diesel engine, can operate as needed completely independently of the operating condition of the engine, can provide a reducing exhaust gas to a NOx storage catalytic converter when necessary for initiating catalyst regeneration, without requiring switching of the engine to rich operation, can quickly raise the temperature of of a NOx storage catalytic converter to a peak catalyst, temperature of 300° C. to 400° C. as needed for catalyst regeneration, even immediately following cold startup of an engine, can quickly raise the temperature of a particle trap to 500° C. to 600° C. for incineration of particles and clearing of the particulate trap, and overcomes the problem of unstable combustion at low pressures associated with conventional in-tailpipe auxiliary burners.